The invention relates to improvements in the flow properties of high boiling fuel oils, and more particularly relates to fuels comprising residuum or vacuum oils having additives incorporated therein to improve their flow properties.
It is well known that petroleum oils often contain wax which will separate out if the oil is cooled down to below a certain temperature. As the temperature is lowered, the concentration of solid material increases developing a gel caused by the formation of a coherent three dimensional network of wax crystals. As a result of this crystal structure, large amounts of oil are entrained in the crystal lattice of wax, thereby preventing the flow of the oil.
The lowest temperature at which the oil will still flow is generally known as the pour point. In the case of fuels, when the oil temperature goes below the pour point and the fuel is no longer freely flowable, difficulty arises in transporting the fuel through flow lines and pumps, as for example, when attempting to transfer the fuel from one storage vessel to another by gravity, or under pump pressure, or when attempting to feed the fuel into a burner. Additionally, the wax crystals that come out of solution tend to plug fuel lines, screens and filters.
This problem has been well recognized in the past and various additives have been suggested for depressing the pour point of oil. One function of such pour point depressants has been to change the nature of the crystals that precipitate from the oil, thereby reducing the tendency of the wax crystals to interlock and set into a gel.
Recently, various oil-soluble polymers characterized by long linear side chains have become known as flow improvers for oils containing high boiling waxes, e.g., waxy crude oils, residua-containing fuel oils, and flash distillate oils. One group of such polymers described in British Pat. No. 1,215,214 and U.S. Pat. No. 3,447,916 is prepared by condensation reactions of a dicarboxylic acid or anhydride, with a basic material which can be a polyol, polyamine or amino alcohol, together with a monocarboxylic acid. A further improvement of this type of flow improver condensation polymer is described in British Pat. No. 1,215,214, wherein pentaerythritol is the polyol. Similarly, addition polymers having long linear side chains have been described in British Pat. Nos. 1,154,966, 1,161,188 and 1,197,474 as flow improvers for crude and residua-containing fuel oils. The most important of these addition polymers are prepared by polymerizing long chain unsaturated esters, such as copolymers of vinyl acetate and behenyl fumarate, or homopolymers of behenyl acrylate, etc. Another class of addition polymers useful in residua and heavy fuel oils, are those described in South African Pat. No. 70/6785, and published German Application No. 1,940,944. Here, long chain alpha olefins are copolymerized with maleic anhydride, which copolymer is then further reacted with long straight chain alcohols or amines. Still another class are polymers and copolymers of straight chain C.sub.18 to C.sub.40 alpha monoolefins, for example, see British Pat. No. 1,267,604 which shows their use in crude oil and French published patent application No. 6,931,526 which shows their use in residuum.
It is also known that some asphaltene fractions can modify the crystal habit of wax separating from wax solvent systems and, in some cases, lower the pour points of such systems. See, for example, B. F. Birdwell, Thesis, University of Texas, 1964, entitled "Effects of Various Additives on Crystal Habit and Other Properties of Petroleum Wax Solutions." This document is available from University Microfilms, Ann Arbor, Michigan.